garphield
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Oleum using HPO3 as a dehydrating agent
I read Magpie's procedure for dehydrating orthophosphoric acid (H3PO4) to metaphosphoric acid (HPO3) via heating to red heat, and might do it myself
soon, but want to ask about a few modifications I am thinking of making. Firstly, since hot phosphoric acid etches glassware, I would like to use a
normal glass jar instead of a borosilicate beaker to heat the phosphoric acid in, and then smashing it with a hammer before grinding the HPO3 in a
mortar and pestle, then adding the powder to sulfuric acid, instead of pouring sulfuric acid into the molten HPO3 (seems very unsafe). After this, I
would ideally vacuum filter the oleum off the solid ortho/pyro/metaphosphoric acid particles (since an excess of the dehydrating agent is used, it
won't get more diluted than H3PO4, which is a solid at room temp) instead of distilling the oleum off. This is because I don't want to risk damaging
my glassware, and also because I would rather not distill oleum. The oleum would probably be contaminated with some dissolved phosphoric acid, but
this shouldn't be a problem for most reactions. Are there any glaring problems with this or should I go ahead and try it?
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Maurice VD 37
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Why do you think the solution of metaphosphoric acid in sulfuric acid deserves to be called oleum ?
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garphield
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http://www.sciencemadness.org/talk/viewthread.php?tid=78495
HPO3 is a strong enough dehydrating agent to dehydrate sulfuric acid, at least when distilled from it. I was asking if the same would be true if you
just filtered it off - I don't know if it would actually be oleum or just a solution of metaphosphoric acid in sulfuric acid, and the distillation is
necessary to drive off the small amount of SO3 that exists in equilibrium. Alternatively, the equilibrium could be in favor of SO3 - I don't know,
which is why I asked.
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Bedlasky
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Why not? Oleum is solution of SO3 in H2SO4, no matter to concentration. Metaphosphoric acid dehydrates sulfuric acid to form sulfur trioxide, which
dissolves in excess of sulfuric acid. So it is technically oleum, even it is impure due to phosphoric acids contamination.
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